Accurate sensing of intracranial electrical activity, such as for determining epileptogenic foci, mapping brain function or otherwise, often may require use of a plurality of brain contacts. Epileptogenic mapping is one example of the use of electrical devices with tissue-engagement contacts. Examples of two kinds of intracranial electrical contact devices are depth probes and flexible flat surface members.
Depth probes, which may be referred to as “depth electrodes”, penetrate deep into the brain tissue. On the other hand, flexible flat surface members, including what are sometimes referred to as “strip” electrodes and “grid” electrodes, may be placed subdurally in direct contact with brain tissue at the surface of the brain.
Examples of such electrodes include but are not limited to electrodes described in U.S. Pat. No. 4,735,208 (Wyler et al.), U.S. Pat. No. 4,805,625 (Putz), U.S. Pat. No. 4,903,702 (Putz), U.S. Pat. No. 5,044,368 (Putz) and U.S. Pat. No. 5,097,835 (Putz).
Each of these different kinds of intracranial tissue-engagement electrodes is connected to some circuitry which typically captures and records the EEG (electroencephalography) signals for analysis of various types. There is a diagnostic need for an increased number of electrodes in order to increase the precision of analysis and diagnosis based on the captured EEG information. An increase in the number of electrodes requires higher data transmission bandwidths if the full amount of data captured from the electrodes is delivered to the monitoring system electronics. Further, there is a diagnostic need to monitor patients for longer periods of time, again for increased precision of diagnosis.
State-of-the-art monitoring systems in diagnostic use, or at least the great majority of such systems, today require a patient suffering from epilepsy to have at least one opening through the patient's skin during the entire period in which the electrodes are implanted for monitoring purposes. It is highly desirable medically, however, to avoid wires/devices through the skin to be in place during such monitoring, since any opening in the skin is an opportunity for infection to develop. Thus, it is highly desirable to avoid prolonged periods during which there are openings in the skin. Further, monitoring systems typically require that a patient be tethered by wires to the monitoring equipment. The existence of a tether is both interfering and inconvenient.
Monitoring systems with electrodes placed to abut brain tissue in various configurations can also be used to provide electrical stimulation of brain tissue as one mode of medical treatment. In making EEG measurements or delivering such electrical stimulation, there is a need to know over time what the condition of an implanted electrode is while it abuts brain tissue. An electrode may shift from its initial implanted position or lesions may form at an electrode, thereby changing conditions under which monitoring and/or stimulation occur in a way which affects the degree of precision of such monitoring and/or stimulation. Knowing and reacting to the electrical impedance of implanted electrodes and to impedance changes would be important to assessment of the condition of an implanted electrode and to enhancing the precision of diagnosis and treatment. One aspect of the present invention provides such impedance-measuring capability.
In order to meet some of the above-mentioned needs, in particular the need to avoid openings in the skin for prolonged periods of time, it is desirable to implant not only the electrodes but also the circuitry which interfaces the electrodes with the remainder of the monitoring system. In order to accomplish this, power must be provided to such an implanted device. This creates a need for low power consumption and medically-safe approaches to providing power.
Power can be provided to an implanted device by either providing an onboard source of power such as a battery or by transmitting power to the implanted device. In the case of devices to which power is transmitted, there is a need to transmit power with as low power loss as possible in order to affect human tissue as little as possible. In the case of providing onboard power, there is a need for completely dry and fully-encased power sources.
Some wireless brain-interactive systems have previously been disclosed in the literature for various purposes, including for monitoring EEG signals in patients. Prior wireless systems for monitoring EEG signals in patients fall short of meeting the wide variety of needs which characterize the complex challenges facing the medical community in providing such diagnosis and treatment today.
Certain other wireless brain-interactive systems in the literature are directed toward providing a brain-machine interface to allow amputees to control prosthetic limbs, an entirely different application than brain monitoring/mapping of neurological-disorder patients. Unlike monitoring/mapping systems, which seek to capture signals that are composites of the signals from a great many neurons, brain-machine interface systems are typically concerned with capturing signals from individual neurons. Significant differences exist in the nature of the electrodes of these two sorts of systems, as does the type of processing that would be required and the bandwidths involved. With regard to the electrodes, brain-machine interface systems have point-like brain-tissue contact, while monitoring/mapping systems employ surface contact, either involving flat disk surfaces or cylindrical surfaces. The present invention is in the specific field of monitoring/mapping systems. The solutions used in brain-machine interface are also more concerned with detecting a time-stamp of activity and transmitting this time-stamp rather than the continuously varying brain signal.
The wireless system for monitoring/mapping brain tissue disclosed herein meets the complex set of needs in the monitoring/mapping field. Among the needs addressed are the need for an increased number of electrodes with all of the data available in the EEG signal at each electrode being captured with high precision in real-time and available for analysis, and the need to assess the condition of the implanted electrodes over time to assure proper capturing of EEG signals on the surface electrodes used.